A number of systems are known for ion beam processing of a workpiece. Among these, ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems include an ion source for converting a gas or solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated and/or decelerated to a desired energy and is directed onto a target plane. The beam may be distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1,000 Angstroms and may eventually require junction depths on the order of 200 Angstroms or less.
The implanted depths of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. However, ion implanters are typically designed for efficient operation at relatively high implant energies, for example in the range of 20 keV to 400 keV, and may not function efficiently at the energies required for shallow junction implantation. At low implant energies, such as energies of 2 keV and lower, the ion beam expands as it is transported through the in implanter, and the beam current delivered to the wafer is much lower than desired. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication costs and is unacceptable to semiconductor device manufacturers.
In an ion implanter, an ion beam is extracted from an ion source, is accelerated and/or decelerated to a desired energy and is delivered to a wafer. In extracting ion beams with a low final energy from the ion source, it is known that more ion current can be extracted if large acceleration and deceleration voltages are used. This tends to increase the virtual image size of the beam and to decrease beam divergence. Deceleration to the final energy may occur at one of several locations along the beamline. The location is selected to limit ion beam expansion and energy contamination.
In an electrode system for an ion source, the final electrode is preferably large compared to the acceleration electrode, as shown in FIGS. 5-9 of a paper by Hiroyuki Ito and Neil Bryan, “Low Energy Beam Extraction in Terms of Magnetic Field, Electric Field and Ion Optics,” IEEE (1997), pages 383-386. However, experimental data shows that when the aperture of the deceleration or final electrode is larger than the acceleration electrode aperture, the acceleration electrode current becomes large, and operation becomes “glitchy,” i.e., the extraction and deceleration gaps are prone to arcing. It is believed that the large current to the acceleration electrode is due to thermal ions from the beam plasma formed after the electrode system. The thermal ions are pulled from the beam plasma by the large fields on axis near the last deceleration electrode.
U.S. Pat. No. 5,196,706, issued Mar. 23, 1993 to Keller et al. discloses an extractor and deceleration lens for ion beam deposition apparatus. U.S. Pat. No. 5,932,882, issued Aug. 3, 1999 to England et al. and U.S. Pat. No. 5,969,366, issued Oct. 19, 1999 to England et al. disclose ion implanters with post mass selection deceleration. U.S. Pat. No. 5,747,936, issued May 5, 1998 to Harrison et al. discloses ion implantation apparatus with improved post mass selection deceleration.
All of the known prior art systems for producing low energy ion beams have had one or more disadvantages, including high electrode currents, glitchy operation and high beam divergence. Accordingly, there is a need for improved methods and apparatus for producing low energy ion beams.